The present invention is directed to a load shifting control system and, more particularly, to a control system for selectively allocating a plurality of discrete electrical loads between a first power source and a second power source.
Electrical utility companies frequently approach and occasionally surpass their maximum generating capabilities during peak demand periods. Consequently, these utilities commonly assess seasonal demand penalties based on each customer's peak level of usage during the billing period. In response, electrical utility customers have employed a number of practices to avoid such penalties. Both commercial and residential customers can reduce or "shave" their peak usage and thus their peak demand charge by shifting some of their peak demand to non-peak time periods, by eliminating unnecessary peak demand altogether or by shifting some of their peak demand to a secondary power source such as a generator or fuel cell.
Supermarkets are among the electrical utility customers which stand to benefit from shifting part of their peak demand to a secondary power source. The domestic supermarket industry accounts for approximately 4% of all electrical energy consumed in the United States. Moreover, the industry relies almost exclusively upon electrical energy to power its refrigeration systems, which account for about half of the total supermarket energy consumption. Furthermore, most supermarkets have a relatively low marginal income (less than 1% of sales) and are thus particularly motivated to reduce operating expenses (e.g., energy consumption) as a percentage of sales.
In the summertime, for example, peak usage generally occurs on weekday afternoons when the demand for air conditioning and refrigeration is greatest. However, natural gas consumption and pricing are relatively low during summer months when heating requirements are limited. Therefore, load shifting using natural gas as an alternate energy source not only reduces costs for utility customers but also benefits both electrical utilities (by reducing peak capacity requirements) and suppliers of natural gas (by increasing demand during summer months).
Many utility customers, and nearly all supermarkets, have access to a secondary source of power such as an on-site generator purchased for use as an emergency power source in the event of a power outage. A privately-owned secondary power source may also be used to supplement the power purchased from an electrical utility. While a customer's peak demand charge can be reduced by shifting part of the peak demand to a secondary generator, the customer must closely compare the cost of generating power with the cost of purchasing power from the electrical utility to maximize net savings because these costs can vary dramatically from hour to hour. In fact, if the customer does not closely monitor these costs, a reduction in the customer's peak demand charge could be negated, or the overall cost could be greater than if no supplemental power were used.
As an example, a supermarket could satisfy its peak demand by supplementing the power it purchases from the electrical utility with a secondary generator (e.g., a synchronous generator or an inductive generator) operated in parallel with the utility and controlled to produce only the amount of power necessary to maintain the supermarket's peak demand at or below a predetermined value. In other words, the supermarket could employ a single power distribution network in which two parallel power sources entirely service a single load (i.e., the supermarket's entire energy requirements). Furthermore, the supermarket could provide one or more additional generators to entirely service the needs of a particular discrete portion of its energy requirements. That is, the supermarket could employ one or more additional power distribution networks each having a generator (other than the one in parallel with the utility) which is dedicated to and entirely services a particular portion of the load.
However, there are several important drawbacks and limitations associated with the example discussed above. First, parallel operation of synchronous generators is potentially dangerous because it is possible to electrocute utility workers if there is a power outage. Second, reducing the danger associated with parallel synchronous generators may be relatively expensive because it requires the purchase and installation of costly circuit protection hardware such as isolation switch gear. Further, inductive generators have the significant drawback that they are not capable of operating as standby-by power sources. Moreover, the approach of using a synchronous generator to service only a particular load(s) lacks flexibility in that it involves single load power distribution networks which cannot dynamically load and unload a secondary power source in response to changing economic factors. In contrast, by combining multiple discrete loads to create relatively small generator loading steps, it is possible for a control system to dynamically load and unload a generator to maximize net savings. Thus, there is a need for a less cumbersome and less expensive load shifting control system for reducing a utility customer's peak demand by dynamically allocating a plurality of discrete loads between two or more power sources which are independent of and isolated from one another.
As mentioned above, a number of economic factors may dictate the manner in which a utility customer allocates its load between a utility and a secondary power source such as a generator. When the energy charge (i.e., dollars per kilowatt-hour) of the electric utility is less than an on-site power source, the entire load should be connected to the utility except for peak shaving purposes (i.e., if the utility assesses a peak demand charge). By contrast, when the cost of generating electrical energy (i.e., KW-hours) is lower than the cost of purchasing it from the public utility, the customer's load should be shifted to the generator up to the maximum load capacity of the generator except to the extent certain factors dictate otherwise (e.g., where reducing the generator run time maximizes net savings due to lower maintenance costs). In many geographic areas, the relevant economic factors vary considerably over time such that the electric utility is the less expensive energy source sometimes, and the generator is the less expensive energy source other times. Accordingly, there is a need for a load shifting control system capable of periodically evaluating the relevant economic factors and selectively shifting a utility customer's discrete loads between at least two power sources in response to changing economic factors. In this regard, it is important to know the actual demand associated with each of the discrete loads because the actual demand will not always equal the "rated" demand assigned to a particular load. Thus, there is also a need for a load shifting control system capable of accurately determining the actual demand associated with each of the discrete loads.